N. Frage

Laser produced functionally graded tungsten carbide coatings on M2 high-speed tool steel

Abstract The objective of the investigation was to produce functionally graded, carbide alloyed multilayer coatings on M2 high-speed steel by laser alloying with direct injection of WC powder into the melt pool. Single layer coatings with a wide alloying range corresponding to 12–58 wt.% W and 1.3–4.3 wt.% C, respectively, were produced by varying laser beam power and beam traverse velocity. Depending on the alloying degree, four different types of structures were observed in laser alloyed coatings; they were characterized by scanning electron microscopy and X-ray microanalysis. Multiple laser alloying with beam power decreasing at each successive stage was used for producing a triple-layer coating with tungsten content increasing from layer to layer and reaching 75 wt.% in the upper layer. The observed hardness was in the 1100–1200 HV range for single layer coatings with 40–50% W and as high as 1600 HV in the upper layer of a triple coating with 75% W. The coating with 58 wt.% W showed wear resistance five times as high as compared with the unalloyed laser-melted M2 steel.

Wetting of TiC by non-reactive liquid metals

Abstract This paper is concerned with the wetting of TiC substrates by the non-reactive metals ( Me=Cu, Ag, Au and Sn ). The contact angle in the TiC/Sn and TiC/Cu systems reaches an equilibrium value slightly below 90° owing to the partial dissolution of titanium carbide in the molten metals. The intrinsic non-wetting behavior in the TiC/Ag system was confirmed. In the TiC/Au system, the non-wetting behavior is attributed to the strong metal–ceramic interaction that takes place at the interface and leads to the formation of a graphite layer. The addition of Fe or Ni increases the solubility of carbon in molten Au and decreases considerably the contact angle by preventing the formation of a graphite layer at the interface. Alloying non-reactive metals with Ti was done either by direct addition to the melt or by transfer from a sub-stoichiometric titanium carbide substrate. In both instances, the presence of Ti in the liquid metal, irrespective of its origin, improves wetting dramatically. The specific wetting behavior of the different Me–Ti alloys are well accounted for by the basic thermodynamic properties of the respective Me–Ti systems and of the titanium carbide phase.

A novel approach for the preparation of B4C-based cermets

Abstract Additions of TiO2 affect significantly the sintering behavior of B4C. The two powders react at approximately 1500°C according to the reaction: B4C+TiO2→B4C1−x+TiB2+CO↑ (or CO2↑). Above 2000°C, the resulting two-phase, B4C1−x+TiB2, mixture sinters at a higher rate than the single-phase boron carbide. The rate of sintering increases dramatically for mixtures that contained initially 40 wt% TiO2, yielding after sintering at 2190°C for 1 h a 95% dense, fine-grained composite material, consisting of sub-stoichiometric B4C1−x and TiB2. The boron-rich carbide displays very high affinity to carbon and reverts rapidly to stoichiometric composition when treated at high temperature in a carburizing atmosphere. The two-phased ceramic can be sintered to various levels of density and infiltrated with a molten metal. After infiltration with molten aluminum, the hardness of the resulting cermets ranges from 800 to 2500 HV and the flexural strength varies from 750 to 350 MPa depending on their ceramic-to-metal ratio.

Wetting phenomena in the TiC/(Cu-Al) system

The wetting behavior in the TiC/(Cu–Al) system was studied by the sessile drop method over the entire Cu–Al alloy concentration range. The experimental results show that the wetting evolution in the TiC/Cu system is controlled by partial dissolution of the titanium carbide phase. The presence of oxygen on the TiC surface strongly inhibits the interaction between the ceramic and molten Cu, thus leading to non-wetting conditions. The improved wetting of the oxygen-free TiC substrate by Cu–Al alloys is due to the enhanced transfer of titanium from the carbide phase into the melt, and results from its increased solubility in the Al-containing molten alloy. The wetting of TiC substrates covered with an oxygen-containing layer by a molten Cu–Al alloy is affected by the reduction of this layer and transfer of the released Ti into the molten metal. Enhanced wetting may also result from in situ deoxidization of the surface of the Cu–Al liquid drop and of the TiC substrate due to the evaporation of the Al2O sub-oxide at elevated temperature. A thermodynamic analysis of the systems under consideration is in good agreement with the experimental observations.

Iron-titanium-carbon system. I. Equilibrium between titanium carbide (TiCx) of various stoichiometries and iron-carbon alloys

This paper presents the computational results of the thermodynamic equilibrium in the ternary system Ti-Fe-C, at 1,773 K. In another paper these results are applied to explain the interaction between TiC{sub x} and iron carbon alloys, observed during infiltration of a porous titanium carbide body by molten iron. The main result is transfer of carbon from the melt to non-stoichiometric titanium carbide and of titanium from the carbide to the melt, resulting in microstructural changes in the final composite. In order to calculate the composition of the coexisting phases of the Fe-Ti-C system, it is necessary to know the thermodynamic properties of the nonstoichiometric titanium carbide and the ternary liquid solution Fe-Ti-C. Direct calculation of the equilibrium state is not possible, because not all the required data are available at present. However, from the results found in the literature and after making some assumptions, a set of reasonable values can be obtained, from which equilibrium compositions can be estimated.

Iron-titanium-carbon system. II. Microstructure of titanium carbide (TiCx) of various stoichiometries infiltrated with iron-carbon alloy

Titanium carbide (TiC{sub x}) is one of the most suitable compounds for the production of ceramic-metal composite materials containing Ni, Fe and their alloys. Most investigators assume that the stoichiometry of TiC is constant when used for preparation of composites. However, it is important to note that interaction between titanium carbide and the metal matrix during liquid phase sintering or free infiltration of the metal may change the composition of the phases. These changes, and especially those of the carbon content in metal solutions, can drastically affect the properties of the resulting composites. This paper presents experimental results of the interactions between non-stoichiometric titanium carbide and molten Fe-C alloys at 1,773 K, during the preparation of infiltrated ceramic-metal composites. The results are interpreted according to phase equilibrium computations presented in another paper.

Graded ceramic preforms: various processing approaches

Abstract The paper reviews several approaches aimed at generating ceramic preforms with an in-built porosity gradient. Some of these approaches rely on variations of the sinterability in stacked powder layers, e.g., as a function of the carbon content ( x ) in monophased stacked TiC x layers, of the TiC-to-TiB 2 ratio in biphased TiC–TiB 2 ceramics, and of particle size in B 4 C preforms. Graded porosity may be achieved by the controlled mass loss through evaporation of graded amounts of volatile sintering additives (ionic salts) or as a result of the reaction between a base ceramic (B 4 C) and varying amounts of TiO 2 additions. The essential experimental details and the hardness profiles obtained in graded ceramic preforms that had been infiltrated with molten Al are presented.

Graded TiC-based cermets

Two different approaches were used for producing infiltrated, one-dimensionally graded ceramic–metal composites. Graded preforms based on titanium carbide TiCx over a range of composition (0.50<x<0.80) were infiltrated with molten Cu. The resulting hardness decreased from 1100 HV on the carbon-lean carbide edge of the sample to 500 HV on the stoichiometric TiC side, reflecting the increase in the ceramic content at low values of x. In order to check the validity of an approach that relies on a varying interaction with the molten metal, graded TiCx preforms based on a narrow carbon-content range, 0.90<x<0.98 were infiltrated with molten Fe–C alloys. The hardness profile across this graded composite extends from 600 HV at the low carbon content side of the carbide to 1450 HV on the stoichiometric TiC side of a heat-treated sample, and fully reflects the changes induced in the metallic matrix. The present study underlines the potential of ceramic phases that extend over a composition range to form the backbone of graded ceramic–metal composites.

Rim region growth and its composition in reaction bonded boron carbide composites with core-rim structure

Aluminum was detected in reaction-bonded boron carbide that had been prepared by pressureless infiltration of boron carbide preforms with molten silicon in a graphite furnace under vacuum. The presence of Al2O3 in the heated zone, even though not in contact with the boron carbide preform, stands behind the presence of aluminium in the rim region that interconnects the initial boron carbide particles. The composition of the rim corresponds to the Bx(C,Si,Al)y quaternary carbide phase. The reaction of alumina with graphite and the formation of a gaseous aluminum suboxide (Al2O) accounts for the transfer of aluminum in the melt and, subsequently in the rim regions. The presence of Al increases the solubility of boron in liquid silicon, but with increasing aluminum content the activity of boron decreases. These features dominate the structural evolution of the rim-core in the presence of aluminum in the melt.

Effect of impurity levels on the structure of solidified aluminum under pulse magneto-oscillation (PMO)

The effect of pulse magneto-oscillation (PMO) treatment on the solidified structure and the cooling curves of pure aluminum samples of two grades were investigated. The PMO treatment leads to decrease in the supercooling level and to refinement of the solidified structure. The effect of the PMO treatment strongly depends on the purity level of aluminum. A model is proposed to explain the effect of the PMO treatment on the structure of solidified metals.